Airway Epithelial Nucleotide Release Contributes to Mucociliary Clearance

Abstract

Mucociliary clearance (MCC) is a dominant component of pulmonary host defense. In health, the periciliary layer (PCL) is optimally hydrated, thus acting as an efficient lubricant layer over which the mucus layer moves by ciliary force. Airway surface dehydration and production of hyperconcentrated mucus is a common feature of chronic obstructive lung diseases such as cystic fibrosis (CF) and chronic bronchitis (CB). Mucus hydration is driven by electrolyte transport activities, which in turn are regulated by airway epithelial purinergic receptors. The activity of these receptors is controlled by the extracellular concentrations of ATP and its metabolite adenosine. Vesicular and conducted pathways contribute to ATP release from airway epithelial cells. In this study, we review the evidence leading to the identification of major components of these pathways: (a) the vesicular nucleotide transporter VNUT (the product of the SLC17A9 gene), the ATP transporter mediating ATP storage in (and release from) mucin granules and secretory vesicles; and (b) the ATP conduit pannexin 1 expressed in non-mucous airway epithelial cells. We further illustrate that ablation of pannexin 1 reduces, at least in part, airway surface liquid (ASL) volume production, ciliary beating, and MCC rates.

Keywords: ATP release; airway epithelia; mucociliary clearance; pannexin 1; vesicular nucleotide transporter.

Figure 1

Purinergic regulation of mucus hydration and clearance The A_2BR imparts cyclic AMP-regulated CFTR Cl⁻ channel activity, while the P2Y₂R promotes Cl⁻ secretion via CaCC/TMEM16 and inhibits Na⁺ absorption. The P2Y₂R also promotes CFTR-mediated Cl⁻ secretion PKC as well as Ca²⁺-promoted cyclic AMP formation.

Figure 2

ATP release pathways in airway epithelia. Cytosolic ATP is released from ciliated cells via the plasma membrane channel PANX1. VNUT transports cytosolic ATP into Goblet cell mucin granules (MG). ATP and its metabolites within MG are secreted concomitantly with mucins.

Figure 3

Pannexin 1 deletion reduces MCC activities. (A) ASL height was measured in MTE cells from WT and Panx1 KO mice, as described in Methods. Twelve measurements were performed per culture and 4 cultures were used per condition. The data are representative of two independent experiments; mean ± SD. (*) p < 0.05, Student’s t-test (Sigma Plot). (B): CBF was assessed in tracheas in situ after retracting the muscle and connective tissue. Exposed tracheas were lit with a DC red light and CBF was recorded under a dissecting microscope outfitted with a digital camera interfaced with Basler software; mean ± SD, n = 4–6. (**) p < 0.01, two-way ANOVA (Sigma Plot). (C): MCC was measured in 10 WT and 11 Panx1 KO mouse tracheas, in vivo. (**) p < 0.01, Student’s t-test.